The juxtaglomerular apparatus (JGA) represents a major structural component of the renin-angiotensin system, and is one of the most important regulatory sites of renal salt & water conservation and blood pressure maintenance. The process of glomerular filtrate formation and regulation of renal hemodynamics, including the tubuloglomerular feedback (TGF) mechanism from the macula densa (MD) and renin release, involves the complex interaction of a number of different cell types of the JGA. The mechanism of these two major JGA functions, the MD control of TGF and renin release, appear to be complex and are unresolved in several areas. Very recently, we established a new experimental model that utilizes multiphoton imaging (Peti-Peterdi et al. Two-photon excitation fluorescence imaging of the living juxtaglomerular apparatus. Am J Physiol Renal Physiol 283: F197-F201, 2002) to visualize morphological changes of the living JGA in response to variations in tubular fluid composition. This approach has several advantages over earlier methods, since complex mechanisms, in a single isolated JGA, can be examined in real-time, with high temporal and spatial resolution. This can be performed on the individual cell level, and is free of systemic influences. In preliminary experiments we made several novel observations, including tubular NaCI/osmolality dependent MD cell swelling, glomerular shrinkage and identified a sphincter in the terminal, intraglomerular part of the afferent arteriole. Also, to our great surprise, these responses were blocked by the inhibition of the MD apical Na:H exchanger NHE2 isoform, but not the furosemide sensitive Na:2CI:K cotransporter. In addition, renin expression is greatly increased in NHE2 knockout mice. This new technology will be used in our studies to functionally and morphologically analyze ionic transporters, and intra-, and intercellular signaling mechanisms in various cell types in the JGA that constitute the TGF and renin release mechanisms. These studies should yield clinically important information since TGF and the JGA-associated renin-angiotensin system are major regulators of renal hemodynamics and participate in control of body fluid and electrolyte balance and blood pressure regulation.